Using Exergetic EfficienciesRefrigerant 134a enters a counterflow heat exchanger operating...

Using Exergetic Efficiencies

Refrigerant 134a enters a counterflow heat exchanger operating at steady state at −20°C and a quality of 35% and exits as saturated vapor at −20°C. Air enters as a separate stream with a mass flow rate of 4 kg/s and is cooled at a constant pressure of 1 bar from 300 to 260 K. Heat transfer between the heat exchanger and its surroundings can be ignored, as can the effects of motion and gravity.

(a) As in Fig. E7.6, sketch the variation with position of the temperature of each stream. Locate T0 on the sketch.

(b) Determine the rate of exergy destruction within the heat exchanger, in kW.

(c) Devise and evaluate an exergetic efficiency for the heat exchanger.

Let T0 = 300 K, p0 = 1 bar.

Fig. E7.6